Particle Questions that include Feynman Diagrams

Q8. A positron is emitted from a nucleus when a proton changes to a neutron in the nucleus.

The Feynman diagram for the quark interaction is shown in the diagram.

(a) Identify the particles labelled A, B, C and D in the diagram.

Remember that for the AQA syllabus you are only doing proton to neutron and vice versa - therefore the up will go to down and vice versa!

A = d (down quark)

The up quark ispositive - therefore positive charge goes into the junction - the down quark path is negative - therefore the positive charge must go out via the boson... it is a weak interaction - therefore B is a w+ boson.

B = w+

The two products were both needed for the final mark.

We have positive charge coming out ot the boson so there must be a positron rather than an electron. The positron is an antiparticle so the other lepton must be a neutrino - in fact as electrons rather than tau or muon leptons are involved we should state it is an electron neutrino.

C and D (either way round) = e⁺ (positron) and ν_e

(3 marks)

(b)

(i) State the interaction responsible for this process.

weak

(1 mark)

(ii) State which letter in the diagram above represents an exchange particle.

B (they ask for the letter in the diagram - not your answer!)

(1 mark)

(iii) State one difference between this exchange particle and a photon.

The photon is the exchange particle for the electromagnetic force - they are both exchange particles. When asked for difference between two things you need to state the properties of both and you must clearly state which one has which property. Answers such as 'It has charge' get no marks! This is A level not GCSE!

The w-boson has charge whereas a photon does not have charge.

The w-boson has (rest) mass but the photon has zero (rest) mass.

The photon has an infinite range but the range of the w+ boson is much shorter.

(1 mark)

(c) Energy and momentum have to be conserved in this process.

State two other quantities that need to be conserved and show that they are conserved in the process.

You have to SHOW they are conserved. Therefore you have to write out the equation and fill in the values.... Waffling on about them being balanced is pointless (or rather markless!) - it also takes an age to do....

As strangeness is on neither side of the reaction it was not counted. They are looking for quantities to be balanced not zero values

You got one mark for each balanced quantity (max 2 marks) and one mark for clearly demonstrating they balance (max 2 marks)

 

	u	→	d	+	e+	+	ve
charge	+2/3	→	-1/3	+	1	+	0	balances
lepton number	0	→	0	+	-1	+	+1	balances
baryon number	+1/3	→	+1/3	+	0	+	0	balances

 

(4 marks)

(Total 10 marks)